Evolution and development of polyacid-modified adhesives for lased dentin

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This study was to investigate 1) the changes of surface characteristics of Er : YAG or CO2 lased dentin and 2) the adhesive properties of resinous materials to the lased dentin. From Scanning Electron Microscopic (SEM) and Atomic Force Microscopic (AFM) study, the smear layer could not be detected on the lased dentin surface, and the structural defects such as micro cracks and stratiform structure were formed. Thermographic analysis exhibited that the laser irradiation generated high heat of temperature value of above 200 ℃. The analysis of lased dentin using Energy Dispersive X-ray Spectroscopy, Electron Probe X-ray Microanalyser, and Nano-indentation Tester revealed the decrease in hardness and in concentration of Ca and P in the superficial layer of approximately 30 μm thickness. According to these changes heat-denatured layer and a layer with destroyed dentinal collagen fibers were observed under Light Microscopy (LM), Transmission Electron Microscopy (TEM) and X-ray photoelectron Spectroscopy.Early bond strength and durability of adhesion of the commercial adhesives to the lased dentin were significantly lower than those to the rotary cut dentin in micro-tensile bond test. This might be caused by the concentration of stress to those structural defects, by the inhibition of resin-impregnation due to the denatured layer, or by the deterioration of mechanical properties of the hybrid layer and the lased dentin itself. In order to adapt commercial adhesives to lased dentin, pretreatments for the removal of defects and denatured layer were examined. LM, SEM and TEM study revealed that they could be removed with 30 seconds' phosphoric acid etching followed by 60 seconds' (Er : YAG) or 120 seconds' (CO2) sodium hypochlorite treatment. This pretreatment could regain adhesive properties of commercial bond systems equally to those to rotary cut dentin.